Motor Type Selection

maxon precision motors, inc. Posted 10/13/2011

Parameters that define a motor type are the mechanical output power, the shaft bearing system, the commutation system used, and the possible combinations with gearheads

and sensors. The most important criteria include the speed and torque requirements, and the commutation system.

Some of the formulas used in this article use subscripts that need to be explained. The variables on the motor shaft (output) are identified with the subscripts Mot. For example, n_Mot stands for the required motor speed. Parameters that describe the characteristics of the motor have no special additional subscript. For example, n_o stands for the no-load speed of the motor.

We’ll discuss speed and torque requirements first. The maximum speed that occurs on the motor shaft n_maxMot should be below the maximum permissible speed of the motor n_max, or n_maxMot < n_max.

As a rule, expected useful life decreases as motor speed increases because of the greater load on the bearings and the higher mechanical wear of the brush system on DC motors. Noise generation increases as well. These effects are particularly pronounced at speeds above the maximum permissible speed. The effective torque M_rmsMot that is required must be less than the rated torque of the motor M_N, or M_rmsMot< M_N.

Rated torque is derived from the rated current I_N, which is selected to equal the maximum continuous current of the motor (from a thermal point of view). Note that the low friction losses of maxon motors are included in the permissible continuous torques of the motors. The friction losses of the graphite brushes and the iron losses of brushless EC motors depend on speed. The result is a visible rounded boundary of their continuous operating range.

maxon’s EC 4-pole brushless motors

The motor must be able to produce the maximum torque for the application. This means that during startup the motor’s stall torque at the rated voltage M_H should not be exceeded. On larger motors with a correspondingly higher stall torque, brief peak torques up to approximately four times the rated torque M_N can be handled without any problems: M_maxMot < 4 • M_N or M_axMot < M_H.

A more detailed analysis would take into account the extent and duration of the overload, as well as the winding and ambient temperatures. In very dynamic applications, the additional torque needed for motor acceleration must be included in the calculation of the operating points: M_Mot,_a = J_rot • p/30 • DnMot/Dt_a (duration Dt_a, rotor moment of inertia J_rot). The effective load must then be recalculated.

There are influences on the rated torque. Take, for example, the rated torque in a maxon motor. M_N represents the maximum permissible continuous torque. Under standard conditions at 25^oC the motor reaches the maximum permissible temperature. The rated torque varies as a function of the ambient temperature T_Amb and mounting conditions; a higher ambient temperature results in a less efficient dissipation of heat and consequently a lower continuous torque.

Similarly the details of the thermal coupling have an influence on the thermal resistance R_th2 between housing and ambient and thus on the rated torque. As the heat dissipation improves, like at a lower R_th2, the rated torque of the motor increases. The thermal resistance R_th2 can easily be reduced by one-half by forced cooling or thermal coupling to a heat conducting (metal) heat sink.

maxon’s brushed RE-max motors

The selection of a suitable commutation system is an important issue. The first choice to be made is between mechanical commutation (DC motor with brushes) and electronic commutation (EC motor or brushless DC motor, BLDC). Considerations relating to life expectancy, reliability, simplicity of actuation, and maximum speed play a role in this process. Furthermore, special environmental conditions such as operation in a vacuum or the ability to withstand sterilization must also be taken into consideration. The most important characteristics and differentiating features are presented in the comparative tables 1 and 2.

Comparative Table 1:

Comparison of the basic characteristics of DC and brushless (EC) motors.

	DC motors	EC motors
Operating life	• limited by brush system • typical: 1000-5000 hours • less than 100 hours under extreme loads • over 15,000 hours under favorable conditions	• limited only by bearing system (preloaded ball bearings) • preloaded ball bearings designed for 20,000 hours of operation
High speeds of rotation	• limited by commutation • typically up to approximately 10,000 min^-1 • in individual cases up to 20,000 min^-1	• very high speeds possible on motors with 1 pole pair • typically up to 50,000 min^-1, in individual cases up to 100,000 min^-1
Actuation	• simple, only DC voltage from battery • motor can be operated directly from battery	• electronic commutation system required
Motor connections	• 2 (DC voltage)	• 3 for winding and 5 for Hall sensors 3 for winding (on sensorless motors)
Maximum efficiency	• very high	• high
Loses	• primarily resistance losses in the winding • brush friction with graphic brushes • negligible eddy current losses	• primarily resistance losses in the winding • increasing eddy current losses at high speeds of rotation
Costs	• dependent on the specific model, construction and control system

Comparative Table 2:

Comparison between the characteristics of precious metal and graphite brushes.

	Precious metal brushes with CLL	Graphite brushes
Use for long operating live	• for small motors • for very small currents and voltages in continuous operation	• for larger motors • for higher currents • for frequent peak currents in start-stop and reversing operation
Applications	• fans • simple pumps	• servo drives in operating equipment • feeder systems, robots • drills, screwdrivers
Additional characteristics compared	• lower costs • lower audible noise level • lower losses, lower no-load current	• higher costs • higher audible noise level • greater losses, higher no-load current
Construction	• few parts, simple construction • brushes preloaded	• more complex construction • lead wire for brush contacts • brushes mounted on shafts and pressed against the commutator with spring
Brush material, brush resistance	• carrier material: spring bronze • contact material: silver alloy (some gold alloy) • very low electrical resistance	• graphite with copper added (appro »Back to Top »View All Tech Papers »Visit the Info Center Email This Page Print This Page Find a Product Find a Company Find a Service Become a Member Bookstore Events Home Copyright Information Online Communication Policies of Operation Privacy Policy Site Map Copyright © Robotic Industries Association. Website Design and Development by AIMG 900 Victors Way, Suite 140, Ann Arbor, Michigan 48108 - T: 734.994.6088 - F: 734.994.3338